Reaching out here as a sort of last gasp effort in case someone else has been able to resolve this issue on this or a similar B550/X570 board.
 
I'm running two M.2 Gen 4 SSDs on the X570 FTW, a Team Group Cardea A440 (boot drive) and a Sabrent Rocket 4.0 (storage drive) with my 5800X3D. I'll run CrystalDiskMark 7 every few months to make sure the drives are performing up to their full Gen 4 potential and for a decent bit they were. Lately, however, the Team Group drive (60% full) has fallen victim to the 1000 MB/s write speed cap that seems to impact many Gen 4 SSDs on the B550/X570 chipsets while the Rocket 4.0 (55% full) happily performs to max potential of the drive. The Team Group drive is in the top M.2 slot and the Rocket 4.0 is in the bottom M.2 slot. The read speed of the Team Group drive is still fine and clocks in around the drive's max of 7000 MB/s, it's just the write speed that's slow.
 
I've dug around and found this seems to be a pretty common issue people have had and have tried all the recommended steps (windows settings, updated chipset drivers, optimized the drive, given it idle time to clear out the write cache on the drive, etc) without anything helping. It's a long shot, but has anyone been able to figure this out on this or another B550/X570 motherboard? Many thanks!

I agree with Cool GTX that you need to check and make sure your top slot SSD has the latest SSD firmware. 

timbalistea
Currently running Windows 10.
Updated AMD chipset drivers a week or two ago.
Team Group doesn't have the firmware immediately available but I've asked them for the latest version.
 
We can hope firmware helps, although the drive was performing up to rated speed several months ago. From other discussions it really seems like a greater B550/X570 issue, quite a few people with high end Samsung drives having the same issue. A clean OS install would temporarily solve the issue for them but it would pop back up a couple weeks later.

 
(snip)"A clean OS install would temporarily solve the issue for them but it would pop back up a couple weeks later."

that sounds like a Windows issue & Not a MB issue 
 
One can hope the firmware for the M.2 can address this issue or Microsoft can be talked into finding & addressing the root cause, if it is an OS issue

Would you consider a swap of your M.2 O/S drive SSD to use the Sabrent Rocket instead?

Hey there! It sounds like you're experiencing some issues with your drive write speeds on your X570 FTW motherboard. There could be a few reasons for this, such as outdated drivers or firmware, or potentially a faulty drive. Have you tried updating your drivers and firmware to the latest versions? You could also try testing your drive with a benchmark tool to see if it's performing as expected. If the issue persists, it may be worth contacting EVGA support for further assistance. They may be able to help diagnose the problem and provide a solution. Good luck and I hope this helps!